Thread: KFX Format
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Old 08-13-2015, 12:39 AM   #8
kovidgoyal
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While I dont have a Kindle capable of enhanced typesetting myself, here are my guesses about KFX:

1) It is most likely a modified form of azw6 -- azw6 is a format that appeared a few months ago, where a normal azw3 file gets its high-res images downloaded separately from the text and put into the sdr folder. This was likely a bandwidth saving feature. https://bugs.launchpad.net/calibre/+bug/1429007
Alternately, if they have finally gotten tired of Palm Databases, it could be a modified for of AZK -- their special snowflake for iOS.

2) KFX files are likely to be "pre-rendered" to some extent, such as having hyphenation pre calculated and added to the text as either soft-hyphens or html tags. This is similar to what Kobo does with kepub.

3) I would not be surprised if amazon is encrypting all kfx files regardless of whether the original has DRM or not. This would be consistent with their "boiling frog" approach to closing off their ecosystem. Over the years they have gradually made their ecosystem more and more closed and hoped that if they do it slowly enough people wont notice. Cases in point: No email delivery of azw3, the completely unnecessary personal document category which tries to separate sideloaded books from purchased books, azw3 itself which was a completely unnecessary obfuscation of EPUB. It should be easy to check if kfx files are encrypted -- just do a binary search for strings you know appear in the text (preferably short ones so that soft hyphens dont interfere)

Last edited by kovidgoyal; 08-13-2015 at 12:42 AM.
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